The invention relates to a double clutch transmission and a method for controlling an automated double clutch transmission.
Double clutch transmissions have long been known and are used by preference in motor vehicles. In general, a double clutch transmission has a first transmission input shaft, connected to a first clutch and speed change gears for a first group of gear speeds, a second transmission input shaft connected to a second clutch and speed change gears for a second group of gear speeds, and a shared transmission output shaft. Usually, the first group of gears comprises the odd-numbered forward gears and the reverse gear, while the second group of gears comprises the even-numbered forward gears. In a double clutch transmission of this nature, a shifting operation, which is to say a change from an active gear to the next higher or next lower destination gear, consists of an initial engagement of the destination gear, which can, for example, be accomplished via a suitable gear change sleeve and a synchromesh mechanism associated with the destination gear, and a subsequent overlapping disengagement of the clutch associated with the transmission input shaft for the active gear and engagement of the clutch associated with transmission input shaft for the destination gear, wherein an interruption of tractive power can largely be avoided. Aside from the shifting operations, the transmission of power is accomplished by means of the first and second transmission input shafts in alternation.
Variations and further refinements of the double clutch transmission have been developed to fit specific application requirements and to implement additional functions.
Thus, for example, DE 3,546,454 A1 discloses a double clutch transmission in which the two transmission input shafts can be connected through an additional clutch. In this way, both clutches can be engaged, particularly for a startup process, which makes it possible to increase the available clutch capacity and reduce the thermal load on the clutches. No other use may be made of the additional clutch, for example for a parking lock.
The class-defining patent DE 4,316,784 A1 describes a double clutch transmission that is designed for an electrically driven vehicle and has only two gears.
The speed change gears for the two gear speeds are each rigidly connected to one of the transmission input shafts and the shared transmission output shaft. A gear change is thus accomplished solely by means of the two associated clutches. To implement a parking lock and a so-called hill holder, both clutches are designed so as to be capable of simultaneous engagement. However, a parking lock of this design cannot be used with the double clutch transmissions that are customary in motor vehicles and that have actively engaging clutches and multiple gears for each transmission input shaft.
Also known in the state of the art is a double clutch transmission (EP 0,691,486 A1) that has two transmission input shafts, wherein a multiplicity of gears are associated with one of the transmission input shafts. In a certain shifting state of this double clutch transmission, only the engine prevents the vehicle wheels from turning, which cannot be called a xe2x80x9creal parking lock.xe2x80x9d
Additionally known from the state of the art is a transmission with a transmission input shaft (DE 198 00 880 A1) that has a shifter, wherein two gears are engaged simultaneously to produce a parking lock. However, releasing the parking lock when there is a relatively large load on the simultaneously engaged gears presents a problem in this transmission.
The object of the invention is to further develop this class of double clutch transmission for use in a motor vehicle equipped with an internal combustion engine, providing a simple and cost-effective parking lock and to specify a suitable method for controlling an automated double clutch transmission in accordance with the invention.
The double clutch transmission intended particularly for use in a motor vehicle is equipped with a first transmission input shaft that has a starting gear and is connected to a first clutch, and with a second transmission input shaft that has a driving gear and is connected to a second clutch, and with a shared transmission output shaft. A plurality of gear speeds are associated with at least one of the two transmission input shafts.
In order to implement a parking lock function, in accordance with the invention two of the gears associated with one of the two transmission input shafts are designed to be capable of being engaged simultaneously.
A quite high number of gears makes it possible to adapt the double clutch transmission to the specific characteristics of an internal combustion engine serving as a drive motor whose usable operating range lies within a limited speed range. By means of a simultaneous engagement of two gears associated with one of the transmission input shafts, which is to say an engagement of associated clutches, the transmission output shaft connected to the drive wheels of the motor vehicle is locked by corresponding speed change gears of the engaged gears and the transmission input shaft connected with them, which makes it possible to reliably prevent the motor vehicle from rolling. Thus, without significant additional effort an effective parking lock is implement that is preferably applied by using actively engaged clutches that are disengaged in the quiescent state. This makes it possible to dispense with a separate parking lock, which frequently is designed in the form of a dog clutch lock located on the transmission output shaft.
In the double clutch transmission, the first transmission input shaft can have all starting gears, and the two simultaneously shiftable gears can be associated with the second transmission input shaft. Such an arrangement makes it significantly easier, or may even be necessary to make it possible, to release the parking lock when the gears that are engaged simultaneously as the parking lock are under great load. For instance, if the motor vehicle is parked on a downhill grade using the parking lock, a coasting torque resulting from the downhill component of the vehicle weight is applied to the transmission output shaft through the drive wheels and other components of the drive train. As a result of the different gear ratios of the gears engaged in the parking lock function, and an input-side connection via the shared transmission input shaft, the speed change gears of the lower gear speed are loaded in the coasting direction and the speed change gears of the higher gear speed are loaded in the direction of powered operation. In this way, the torques acting at the associated clutches can be so large that the gears can be disengaged only with unacceptably large actuating forces or cannot be disengaged at all. Thus, it is advantageous for a startup process if a torque opposite to the direction of the load can first be applied to the transmission output shaft via a gear of the other transmission input shaft, which the present invention makes possible. During this process, the gear in question is first engaged and the associated first clutch is engaged. As a result, an increasing torque acts through the transmission output shaft and relieves the load on at least one of the clutches of the two engaged gears acting as the parking lock so that it can be released with a relatively small actuating force. This then also relieves the load on the other clutch so that it too can be disengaged largely without force, with the result that the parking lock is then fully released. In the example described of the motor vehicle parked on a downgrade, the reverse gear is the appropriate gear for compensation of the load. In the case of a motor vehicle parked on an upgrade, the direction of load is reversed, so in this case a forward gear, for example the first gear, is suitable for compensation of the weight-related load. Because they each have a fairly high available torque while simultaneously having a low starting speed, it is preferable to use the starting gears, i.e. first gear and reverse gear, for compensating loads.
The two simultaneously shiftable gears can be engaged manually. To perform the shifting, it is then useful to provide a separate shifting element, for example a parking shift lever. Engagement of the parking lock can be selected by the driver by means of the shifting element and transmitted to the associated clutches of the two gears in question by a transmission mechanism. The advantage of a manual or mechanical shifting of the gears resides in low susceptibility to problems and the possibility for the driver to directly influence the parking lock. In the event that simultaneous shifting of the two gears is not possible, for example because pawls of at least one of the two clutches in question do not align, the driver can allow the motor vehicle to roll a short distance until the two gears can easily be engaged.
However, the two simultaneously shiftable gears can also be engaged automatically, and a parking position of a shift lever or a separate shifting element can be provided to initiate the shifting. Automated shifting of gears, which generally is performed by means of a control unit and actuators that can be controlled thereby, is widely used in double clutch transmissions. It is thus possible to implement a parking lock with little effort, in other words without an additional mechanism, if the two gears in question can be shifted simultaneously by automatic means. Initiation of shifting can take place in a parking position of the associated shift lever in a manner analogous to shift gates of automatic transmissions. In the event that such a parking position is not provided, the shifting process can also take place by means of a special shifting element which can for example be implemented as an additional parking shift lever, a rocker switch or some other form.
The two simultaneously shiftable gears can be adjacent gears. In the customary arrangement of gears, this would be the combination of second/fourth gear or fourth/sixth gear. When adjacent gears are used, the loading of the gears and thus that of the relevant clutches is lower than when the combination of second and sixth gear is used on account of the smaller difference in gear ratios when engaged. Hence, the parking lock can be engaged and disengaged more easily.
The two simultaneously shiftable gears can be predetermined, in which case it is advantageous to use the largest adjacent gears. Particularly in the case of manual or mechanical operation of the parking lock, predetermination of the two gears to be simultaneously engaged is advantageous in order to simplify the mechanics. It is then usually preferred to use the two largest adjacent gears, since the gear ratio difference is the smallest here, and hence shifting capability is best.
In the case of automated shifting of the gears intended for the parking lock function, it is advantageous if the two simultaneously shiftable gears are variably selectable to a large degree. In the case of a six-speed transmission with three gears per transmission shaft, there are three possible combinations of gears for the relevant transmission input shaft to implement the parking lock function. If, for example, a certain combination of gears is specified with highest priority, but one of the two gears temporarily cannot be shifted or would require an unacceptably high actuating force to shift, for instance because of unfavorable pawl alignment of the associated clutch, then instead the gear not initially intended can be shifted.
In addition, sensors can be provided for determining load magnitude and/or for determining a load direction of the simultaneously shiftable gears. Especially in the case of automated shifting of the gears in question, it is advantageous for a startup process to first establish the load magnitude. If it is small enough, the parking lock can be released immediately without taking further precautions. However, if it is relatively large, determination of the direction of load identifies a gear of the first transmission input shaft that can be used to compensate the load before the parking lock can be released.
A safety lock can be provided to prevent the two simultaneously shiftable gears from being engaged when the motor vehicle is moving. This safety lock can be designed to release when the vehicle is stationary so that the parking lock can be engaged only when the vehicle is stationary. In the case of automated actuation of the two gears used for the parking lock function, the safety lock can consist of a suitable safety query in a control program of an associated electronic control device, for example, which makes it possible to prevent engagement of the two gears intended for the parking lock function as long as motion of the motor vehicle in question is detected by a motion detector, which is evaluated in the safety query. The motion sensor can be designed as a rotation sensor, for example, and be arranged on the transmission output shaft. In the case of manual or mechanical actuation of the parking lock, the safety lock could take the form of a spring-loaded locking pin equipped with a solenoid by means of which the relevant transmission mechanism or the actuating element can be locked by the spring in an inoperative position in which the parking lock is released, and can be enabled by means of the solenoid when the vehicle is stationary to permit engagement.
The method for controlling an automated double clutch transmission provides that in a parking lock function, an activation of a parking lock is accomplished through simultaneous engagement of two gears of a transmission input shaft that has no starting gear. As a result of the fact that the starting gears are associated with the other transmission input shaft, it is possible to compensate loads that may result from the weight of the vehicle when it is parked on downgrades and upgrades so as to achieve largely force-free release of the parking lock.
In order to avoid activation of the parking lock while the vehicle is moving, it can first be determined whether the vehicle is moving or stationary. When the vehicle is moving, activation of the parking lock is prevented; in contrast, when the vehicle is stationary, activation is permitted and can subsequently be performed.
For deactivation of the parking lock, the magnitude of the load on the simultaneously engaged gears can first be determined. If a relatively small load is present, the engagement of the gears in question can be released largely free of force so that no further preparations are necessary to release the parking lock.
However, when a relatively large load is present, it is advantageous to determine the direction of the load, and subsequently to engage a starting gear in a direction opposite to the direction of loading, and to engage an associated clutch until the load is adequately small, whereupon the engagement of the gears can be released in a largely force-free manner.
The double clutch transmission and the corresponding method in accordance with the invention are explained in detail below.